Blog Stats

A new engineer came to the office in the first day of his new career in biggest offshore fabrication yard in Indonesia. He met senior engineers whom he didn’t know what was his engineering discipline expertize.

Senior Engineer 1: Are you instrument engineer?

New Engineer : Yes, I am.

Senior Engineer 1 : You should know about control, shouldn’t you?

New Engineer : I think so.

Senior Engineer 1 : Are you familiar with start, stop, Y, delta, coil & contactor? Those are the basic control theory you should understand.

New Engineer : Mmm… you must be graduated from Electrical Engineering, Are you? I need to learn from you about that. I focussed my study in controlling process and I am very lack in controlling electrical.

Senior Engineer 2 : Our electric is also process, you know. It is process electric.

New Engineer : Oh..ya? You must be starting your career in this yard or might be more than 20 years giving service to this yard sir? So then you call electric is process electric.

Above is only compiling conversation which is not happening in one occasion but summary of daily conversation. In my opinion we should aware in term of control which one is process control and which one is electrical control. If you are E&I engineer, you should know both. If you only Instrument Engineer then you should make interface which one is yours, which one is electrical has, and which one is control system has. However if we are a learner we should know all.

Do you say whether below schematic is a process control or electric control or both?

Gambar 1

I prefer to say above schematic is electrical control, it is intended to control motor’s start and stop. If I am purely instrument engineer, I don’t want to deal with this, give it to electrical. However as we are a learner I want to study about that. Mmm..I don’t remember I got it already or I didn’t during my school. Or it might be I didn’t understand before and I forget now.

First things first we should identify L1, L2, L3, what are they? They are 3-phase electrical power 460 VAC to run the motor. It is purely electrical power converted by winding become magnet and mechanic movement. However we won’t discuss about motor rotation in this topic, will we?. Controlling motors means how to cut and connect the power supply feeding to motors. Please have a look above circuit, it was considered as a basic motor control wiring schematic. Do you guys see the control circuit include 24 VAC? Electrical guys often call those wiring above is control cable. We have control cable but where is instrument? No instrument cable above there. Above is purely electrical control system.

Sometime motor control circuit use 220 VAC voltage. The cables are also categorized as control cable, electrical control cable exactly. In which part PLC and instrumentation take part in the motor control? Let’s find out.

The art of electrical control is about push button, coil and contactor. In my opinion it’s not about analog, PID, feedback, feedforward etc, but purely about discrete system. Possibly someday analog system will be applied in electrical system, or might be today it’s been applied already but I don’t know. Let’s back to above schematic we have STOP & START push button, we have CR coil and contactor, we have M coil and contactor, Overload Thermal sensor OL and contactor, and Transformer from 460 VAC to 24 VAC. When you push START button CR coil activate and CR contactor activate, Coil M activate and contactor M activate and the motor gets the power feeding and it is ON. When overload detected by increasing temperature on thermal sensor then the OL contactor will bu cut, M coil deactivate and M contactor open, and motor doesn’t get feeding and OFF. When overload is not being detected and motor is running normally then push STOP button, CR coil deactivate and CR contactor open, M coil deactivate and M contactor open, the motor no longer has power feeding then it is bloody killed and STOP. Push button is kind of button which will be activate when your hand push it and back to normal designated position when your hand leave it.

The motor will interface with process through Safety Instrumented System. The process through transmitter will tell the motor when it shall run and when it shall stop. From the process control side controlling the motors is only about on-off logic, but from the electrical control side it might be more complicated.